1. Field of the Invention
The present invention relates to semiconductor light-emitting devices such as semiconductor lasers and light-emitting diodes using Group III-V nitride compound semiconductors.
2. Description of the Background Art
As semiconductor lasers capable of light emission over a region ranging from blue to ultraviolet that is necessary for greater density of optical disc, research and development has been actively conducted in recent years on nitride semiconductor lasers using Group III-V nitride compound semiconductors such as AlGaInN, which have been put to practical use.
Nitride semiconductor lasers reported to date have, as their active layer structure, a multi-quantum-well structure in which a well layer made of InGaN (InGaN well layer) and a barrier layer made of InGaN (InGaN barrier layer) and having a lower In composition ratio (commonly about 0.02) than the well layer are alternately laminated. As an example of semiconductor lasers having an active layer of this multi-quantum-well structure, Japanese Patent Application Laid-Open No. 10-261838 (1998) discloses a gallium nitride semiconductor light-emitting device.
The In composition ratio (which means “x” in InxGa(1-x)) of the InGaN well layer takes on various values in accordance with an oscillation wavelength or the thickness of the well layer (well thickness). With the oscillation wavelength being 405 nm and the well thickness being 5.0 nm, for example, the value is about 0.12. It is generally known, however, that the value of In composition ratio in fact varies depending on the position in a layer direction (perpendicular to a growth direction) of the well layer, namely the In composition ratio fluctuates considerably along the layer direction of the well layer. The magnitude of the fluctuations can be as small as several nm, and reach as large as several hundred nm.
Such fluctuations in element composition ratio are a property not seen in materials such as GaAs, AlGaAs, AlGaP and AlGaInP that are often used for a well layer in GaAs LDs, or materials such as InGaAs and InGaAsP that are often used for a well layer in InP LDs.
As has been described, in a semiconductor light-emitting devices such as a semiconductor laser having an active layer of multi-quantum-well structure that includes the InGaN well layer and InGaN barrier layer, fluctuations in the In composition ratio occur in the layer direction of the InGaN well layer. This has rendered semiconductor light-emitting devices having sufficiently high differential efficiency unobtainable.